The American Civil War started with a declaration of secession by seven Southern US states. The seven eventually became twelve and named themselves the Confederate States of America. The Southern states were angry about the Northern attempts to prevent the expansion of slavery into western territories.
This video catalogues many of the major battles fought in the four years of the Civil War. Roughly 200,000 people died in action, but the Northern states eventually prevailed on 9 May 1865, resulting in the abolition of slavery and the preservation of US territorial integrity.